Switching control algorithms on detection of exposure of underlying layer during polishing

ABSTRACT

A method of controlling polishing includes polishing a stack of adjacent conductive layers on a substrate, measuring with an in-situ eddy current monitoring system a sequence of characterizing values for the substrate during polishing, calculating a polishing rate from the sequence of characterizing values repeatedly during polishing, calculating one or more adjustments for one or more polishing parameters based on a current polishing rate using a first control algorithm for an initial time period, detecting a change in the polishing rate that indicates exposure of the underlying conductive layer, and calculating one or more adjustments for one or more polishing parameters based on the polishing rate using a different second control algorithm for a subsequent time period after detecting the change in the polishing rate.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to U.S. Application Ser. No.63/036,392, filed on Jun. 8, 2020, the disclosure of which isincorporated by reference.

TECHNICAL FIELD

The present disclosure relates to chemical mechanical polishing, and inparticular to in-situ real time profile control during polishing a stackof adjacent conductive layers on a substrate.

BACKGROUND

An integrated circuit is typically formed on a substrate (e.g. asemiconductor wafer) by the sequential deposition of conductive,semiconductive or insulative layers on a silicon wafer, and by thesubsequent processing of the layers.

One fabrication step involves depositing a filler layer over anon-planar surface, and planarizing the filler layer until thenon-planar surface is exposed. For example, a conductive filler layercan be deposited on a patterned insulative layer to fill the trenches orholes in the insulative layer. The filler layer is then polished untilthe raised pattern of the insulative layer is exposed. Afterplanarization, the portions of the conductive layer remaining betweenthe raised pattern of the insulative layer form vias, plugs and linesthat provide conductive paths between thin film circuits on thesubstrate. In addition, planarization may be used to planarize thesubstrate surface for lithography.

Chemical mechanical polishing (CMP) is one accepted method ofplanarization. This planarization method typically requires that thesubstrate be mounted on a carrier head. The exposed surface of thesubstrate is placed against a rotating polishing pad. The carrier headprovides a controllable load on the substrate to push it against thepolishing pad. A polishing liquid, such as slurry with abrasiveparticles, is supplied to the surface of the polishing pad.

During semiconductor processing, it may be important to determine one ormore characteristics of the substrate or layers on the substrate. Forexample, it may be important to know the thickness of a conductive layerduring a CMP process, so that the process may be terminated at thecorrect time. A number of methods may be used to determine substratecharacteristics. For example, optical sensors may be used for in-situmonitoring of a substrate during chemical mechanical polishing.Alternately (or in addition), an eddy current sensing system may be usedto induce eddy currents in a conductive region on the substrate todetermine parameters such as the local thickness of the conductiveregion.

SUMMARY

According to an aspect, a method of controlling polishing includespolishing a stack of adjacent conductive layers on a substrate,measuring with an in-situ eddy current monitoring system a firstsequence of characterizing values for a first region of the substrateduring polishing, calculating a first polishing rate from the firstsequence of characterizing values, calculating a first adjustment for afirst polishing parameter based on the first polishing rate, detecting afirst change in the polishing rate that meets at least one firstpredetermined criterion that indicates exposure of the underlyingconductive layer, measuring with the in-situ eddy current system asecond sequence of characterizing values for the first region of thesubstrate during polishing upon detecting the change in the polishingrate, calculating a second polishing rate from the second sequence ofcharacterizing values, and calculating a second adjustment for the firstpolishing parameter based on the second polishing rate. The stack ofadjacent conductive layers includes an outer conductive layer and anunderlying conductive layer. The first characterizing values depend onthicknesses and conductivities of the conductive layers in the stackincluding a thickness of the outer conductive layer undergoing polishingin the first region. The second sequence of characterizing values dependon thicknesses and conductivities of remaining conductive layers in thestack including a thickness of the underlying conductive layerundergoing polishing.

In another aspect, a method of controlling polishing includes polishinga stack of adjacent conductive layers on a substrate, measuring with anin-situ eddy current monitoring system a sequence of characterizingvalues for the substrate during polishing, calculating a polishing ratefrom the sequence of characterizing values repeatedly during polishing,calculating one or more adjustments for one or more polishing parametersbased on a current polishing rate using a first control algorithm for aninitial time period, detecting a change in the polishing rate that meetsat least one first predetermined criterion that indicates exposure ofthe underlying conductive layer and calculating one or more adjustmentsfor one or more polishing parameters based on the polishing rate using adifferent second control algorithm for a subsequent time period afterdetecting the change in the polishing rate.

In another aspect, a method of controlling polishing includes polishinga stack of adjacent layers on a substrate, measuring with an in-situmonitoring system a sequence of characterizing values for the substrateduring polishing, and executing a plurality of instances of a profilecontrol algorithm on a controller. The plurality of instances include afirst instance and a second instance having different values for acontrol parameter. The first instance of the profile control algorithmreceives the sequence of characterizing values during an initial timeperiod, and the second instance of the profile control algorithmreceives the sequence of characterizing values during the initial timeperiod and a subsequent time period. For an initial time period one ormore polishing parameters are controlled using the first instance of theprofile control algorithm based on the sequence of characterizing valuesreceived during the initial time period, exposure of the underlyinglayer is detected based on the sequence of characterizing values fromthe in-situ monitoring system, and for the subsequent time period afterdetecting the exposure of the underlying layer the one or more polishingparameters are controlled using the second instance of the profilecontrol algorithm based on the sequence of characterizing valuesreceived during the initial time period and an subsequent time period.

In another aspect, computer program products are operable to perform thecomputational steps of any of these methods. In another aspect, apolishing system includes a platen to support a polishing pad, a carrierhead to hold a substrate against the poilishing pad, an in-situmonitoring system, and a controller configured to perform thecomputational steps of any of these methods.

Implementations of any of the above aspects may include one or more ofthe following features.

Calculating the first adjustment for the first polishing parameter mayinclude calculating a first projected time at which the first regionreaches a target value based on the first polishing rate, measuring athird sequence of characterizing values for a second region of thesubstrate during polishing with the in-situ eddy current system,calculating a third polishing rate from the third sequence ofcharacterizing values, and calculating a first desired polishing ratefor the second region based on the first polishing rate and the thirdpolishing rate to bring the first region and the second region to thetarget value at substantially the same time. Calculating the secondadjustment for the first polishing parameter may include calculating asecond projected time at which the first region reaches the target valuebased on the second polishing rate, measuring a fourth sequence ofcharacterizing values for the second region of the substrate duringpolishing with the in-situ eddy current system, calculating a fourthpolishing rate from the fourth sequence of characterizing values, andcalculating the second adjustment for the first polishing parameterincludes calculating a second desired polishing rate for the secondregion based on the second polishing rate and the fourth polishing rateto bring the first region and the second region to the target value atsubstantially the same time.

The steps of calculating the first polishing rate and calculating thefirst adjustment for a first polishing parameter may be iterated priorto detecting the change in the polishing rate at a first frequency, andthe steps of calculating the second polishing rate and calculating thesecond adjustment for the first polishing parameter may be iteratedafter to detecting the change in the polishing rate at a secondfrequency. The first frequency may be different from the secondfrequency.

A first Preston matrix may be used for calculating the first adjustmentfor the first polishing parameter, and a second Preston matrix may beused for calculating the second adjustment for the second polishingparameter. The first Preston matrix and second Preston matrix mayinclude different values for at least one element at the same row andcolumn in the first Preston matrix and second Preston matrix.

After detecting the first change in the polishing rate, a second changein the polishing rate that meets at least one second predeterminedcriterion that indicates exposure of a further layer below theunderlying conductive layer may be detected. The at least one firstpredetermined criterion and the at least one second predeterminedcriterion may be different criteria. One of the first and secondpredetermined criteria may be a window logic having a bottom exit mode,and another of the first and second predetermined criteria may be awindow logic having a right side exit mode.

The steps of calculating the first polishing rate may be performed usinga first function, and calculating the second polishing rate may beperformed using a different second function. The first function and thesecond function may use running windows of different duration.Calculating the first adjustment for the first polishing parameter mayinclude limiting the first adjustment to a first maximum change, andcalculating the second adjustment for the second polishing parameter mayinclude limiting the second adjustment to a different second maximumchange. Caculating the first polishing rate from the first sequence ofcharacterizing values may use a correlation curve based on a resistivityof a layer from the stack of adjacent conductive layers. Calculating thesecond polishing rate from the second sequence of characterizing valuesmay use the same correlation curve based on the resistivity of the samelayer from the stack of adjacent conductive layers. The layer may be thelast conductive layer from the stack of adjacent conductive layers.

Repeatedly calculating the polishing rate includes may use a correlationcurve based on a resistivity of a same layer from the stack of adjacentconductive layers during the initial time period and the subsequent timeperiod.

The carrier head may include a plurality of independently controllablechambers. The first polishing parameter may be a pressure for one of thechambers, and the one or more polishing parameters may include one ormore pressures for one or more of the chambers.

Certain implementations can include one or more of the followingadvantages. An in-situ real time profile control system can be used whenpolishing a stack of adjacent conductive layers on a substrate. Thesystem can detect exposure of an underlying conductive layer in thestack, and use this as a trigger to modify operation of the profilecontrol system to account for the different layers. Within-waferpolishing uniformity can be improved.

The details of one or more implementations are set forth in theaccompanying drawings and the description below. Other aspects, featuresand advantages will be apparent from the description and drawings, andfrom the claims.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1A is a schematic side view, partially cross-sectional, of achemical mechanical polishing station that includes an eddy currentmonitoring system.

FIG. 1B is a schematic top view of a chemical mechanical polishingstation.

FIG. 2 is a schematic graph of a static formula for determiningsubstrate thickness based on measured signals.

FIG. 3 is a schematic cross-sectional view of a stack of conductivelayers on a substrate.

FIG. 4A is a schematic graph illustrating a first adjustment for a firstpolishing parameter based on the first polishing rate.

FIG. 4B is a schematic graph of detecting a change in the polishing ratethat indicates exposure of the underlying conductive layer

FIG. 4C is a schematic graph illustrating a second adjustment for thefirst polishing parameter based on the second polishing rate.

FIG. 5A is a flow diagram showing an implementation of detecting theexposure of the underlying conductive layer.

FIG. 5B is a flow diagram showing another implementation of detectingthe exposure of the underlying conductive layer.

Like reference symbols in the various drawings indicate like elements.

DETAILED DESCRIPTION

A polishing apparatus can use an in-situ monitoring system, e.g., aneddy current monitoring system, to detect the thickness of an outerlayer that is being polished on a substrate. During polishing of theouter layer, the in-situ monitoring system can determine the thicknessof different locations of the layer on the substrate. The thicknessmeasurements can be used to trigger a polishing endpoint and/or toadjust processing parameters of the polishing process in real time. Forexample, a substrate carrier head can adjust the pressure on thebackside of the substrate to increase or decrease the polishing rate ofthe locations of the outer layer. The polishing rate can be adjusted sothat the locations of the layer are substantially the same thicknessafter polishing. The CMP system can adjust the polishing rate so thatpolishing of the locations of the layer completes at about the sametime. Such profile control can be referred to as real time profilecontrol (RTPC).

For manufacturing of some devices, a substrate to be polished caninclude a stack of multiple conductive layers of different compositions.Each of the layer may have a different conductivity, and/or a differentpolishing rate in response to a given applied pressure. When polishingthrough multiple conductive layers on a substrate consecutively in asingle polishing process, within-wafer non-uniformity (WIWNU) remains achallenge. However, an in-situ real time profile control system canaddress this problem. In particular, a monitoring system can detect apolishing transition when polishing process removes an outer conductivelayer and exposes an underlying conductive layer, e.g., by detecting achange in rate of change of the measured characteristic. This cantrigger a modification of the in-situ real time profile control system,leading to improved polishing uniformity.

FIGS. 1A and 1B illustrate an example of a polishing apparatus 100. Thepolishing apparatus 100 includes a rotatable disk-shaped platen 120 onwhich a polishing pad 110 is situated. The platen is operable to rotateabout an axis 125. For example, a motor 121 can turn a drive shaft 124to rotate the platen 120. The polishing pad 110 can be a two-layerpolishing pad with an outer polishing layer 112 and a softer backinglayer 114.

The polishing apparatus 100 can include a port 130 to dispense polishingliquid 132, such as slurry, onto the polishing pad 110. The polishingapparatus can also include a polishing pad conditioner to abrade thepolishing pad 110 to maintain the polishing pad 110 in a consistentabrasive state.

The polishing apparatus 100 includes at least one carrier head 140. Thecarrier head 140 is operable to hold a substrate 10 against thepolishing pad 110. The carrier head 140 can have independent control ofthe polishing parameters, for example pressure, associated with eachrespective substrate.

In particular, the carrier head 140 can include a retaining ring 142 toretain the substrate 10 below a flexible membrane 144. The carrier head140 also includes a plurality of independently controllablepressurizable chambers defined by the membrane, e.g., three chambers 146a-146 c, which can apply independently controllable pressures toassociated zones on the flexible membrane 144 and thus on the substrate10. Although only three chambers are illustrated in FIG. 1 for ease ofillustration, there could be one or two chambers, or four or morechambers, e.g., five chambers.

The carrier head 140 is suspended from a support structure 150, e.g., acarousel or a track, and is connected by a drive shaft 152 to a carrierhead rotation motor 154 so that the carrier head can rotate about anaxis 155. Optionally the carrier head 140 can oscillate laterally, e.g.,on sliders on the carousel 150 or track; or by rotational oscillation ofthe carousel itself. In operation, the platen is rotated about itscentral axis 125, and the carrier head is rotated about its central axis155 and translated laterally across the top surface of the polishingpad.

While only one carrier head 140 is shown, more carrier heads can beprovided to hold additional substrates so that the surface area ofpolishing pad 110 may be used efficiently.

The polishing apparatus 100 also includes an in-situ monitoring system160. The in-situ monitoring system 160 generates a time-varying sequenceof values that depend on the thickness of a layer on the substrate. Thein-situ monitoring system 160 includes a sensor head at which themeasurements are generated; due to relative motion between the substrateand the sensor head, measurements will be taken at different locationson the substrate.

The in-situ-monitoring system 160 can be an eddy current monitoringsystem. The eddy current monitoring system 160 includes a drive systemto induce eddy currents in a conductive layer on the substrate and asensing system to detect eddy currents induced in the conductive layerby the drive system. The monitoring system 160 includes a core 162positioned in a recess 128 to rotate with the platen, at least one coil164 wound around a portion of the core 162, and drive and sensecircuitry 166 connected by wiring 168 to the coil 164. The combinationof the core 162 and coil 164 can provide the sensor head. In someimplementations, the core 162 projects above the top surface of theplaten 120, e.g., into a recess 118 in the bottom of the polishing pad110.

The drive and sense circuitry 166 is configured to apply an oscillatingelectric signal to the coil 164 and to measure the resulting eddycurrent. A variety of configurations are possible for the drive andsense circuitry and for the configuration and position of the coil(s),e.g., as described in U.S. Pat. Nos. 6,924,641, 7,112,960 and 8,284,560,and in U.S. Patent Publication Nos. 2011-0189925 and 2012-0276661. Thedrive and sense circuitry 166 can be located in the same recess 128 or adifferent portion of the platen 120, or could be located outside theplaten 120 and be coupled to the components in the platen through arotary electrical union 129.

In operation the drive and sense circuitry 166 drives the coil 164 togenerate an oscillating magnetic field. At least a portion of magneticfield extends through the polishing pad 110 and into substrate 10. If aconductive layer is present on substrate 10, the oscillating magneticfield generates eddy currents in the conductive layer. The eddy currentscause the conductive layer to act as an impedance source that is coupledto the drive and sense circuitry 166. As the thickness of the conductivelayer changes, the impedance changes, and this can be detected by thedrive and sense circuitry 166.

Alternatively or in addition, an optical monitoring system, which canfunction as a reflectometer or interferometer, can be secured to theplaten 120 in the recess 128. If both systems are used, the opticalmonitoring system and eddy current monitoring system can monitor thesame portion of the substrate.

The CMP apparatus 100 can also include a position sensor 180, such as anoptical interrupter, to sense when the core 162 is beneath the substrate10. For example, the optical interrupter could be mounted at a fixedpoint opposite the carrier head 140. A flag 182 is attached to theperiphery of the platen. The point of attachment and length of flag 182is selected so that it interrupts the optical signal of sensor 180 whilethe core 162 sweeps beneath substrate 10. Alternatively or in addition,the CMP apparatus can include an encoder to determine the angularposition of platen.

A controller 190, such as a general purpose programmable digitalcomputer, receives the intensity signals from the eddy currentmonitoring system 160. The controller 190 can include a processor,memory, and I/O devices, as well as an output device 192 e.g., amonitor, and an input device 194, e.g., a keyboard. Although illustratedas a single device, the controller 190 can include multiple processorsacross multiple devices, e.g., a networked or otherwise distributedcomputing system.

The signals can pass from the eddy current monitoring system 160 to thecontroller 190 through the rotary electrical union 129. Alternatively,the circuitry 166 could communicate with the controller 190 by awireless signal.

Since the core 162 sweeps beneath the substrate with each rotation ofthe platen, information on the conductive layer thickness is accumulatedin-situ and on a continuous real-time basis (once per platen rotation).The controller 190 can be programmed to sample measurements from themonitoring system when the substrate generally overlies the core 162 (asdetermined by the position sensor). As polishing progresses, thethickness of the conductive layer changes, and the sampled signals varywith time. The time varying sampled signals typically include a sequenceof raw signal values. The measurements from the monitoring systems canbe displayed on the output device 192 during polishing to permit theoperator of the device to visually monitor the progress of the polishingoperation.

The signals, e.g., the sequence of raw signal values, from the eddycurrent monitoring system 160 can be converted into a time-basedsequence of characterizing values, e.g., thickness values. For example,the controller 190 can use a correlation curve that relates the signalmeasured by the in-situ monitoring system 160 to the thickness of thelayer being polished on the substrate 10 to generate an estimatedmeasure of the thickness of the layer being polished. An example of acorrelation curve 203 is shown in FIG. 2. In the coordinate systemdepicted in FIG. 2, the vertical axis represents the value of the signalreceived from the in-situ monitoring system 160, whereas the horizontalaxis represents the value for the thickness of the layer of thesubstrate 10. For a given signal value, the controller 190 can use thecorrelation curve 203 to generate a corresponding thickness value. Thecorrelation curve 203 can be considered a “static” formula, in that itpredicts a thickness value for each signal value regardless of the timeor position at which the sensor head obtained the signal. Thecorrelation curve 203 can be represented by a variety of functions, suchas a polynomial function, or a look-up table (LUT) combined with linearinterpolation. However, in some implementations, the signals could beconverted into other measurements, e.g., conductivity measurements, orthe signals could themselves be used as the characterizing values.

Returning to FIG. 1, in operation, the CMP apparatus 100 can use theeddy current monitoring system 160 and the controller 190 to measure thepolishing progress. In particular, the eddy current monitoring system160 and the controller 190 can monitor the thickness of an outerconductive layer or an underlying conductive layer, estimate a firstendpoint time when the projected thickness of the outer conductive layerin a target region on the substrate meets a target thickness, detect apolishing transition when an underlying conductive layer has beenexposed, estimate a second endpoint time when the projected thickness ofthe underlying conductive layer in the target region on the substratemeets the target thickness, adjust a polishing pressure/rate on acontrol zone at a time of a plurality of adjustment times to bring thethickness of both target zone and control zone to the target thicknessat substantially the same time, and/or determine when the underlyingstop layer has been exposed. Possible process control, polishingtransition detection and endpoint criteria for the detector logicinclude local minima or maxima, changes in slope, threshold values inamplitude or slope, or combinations thereof.

The controller 190 may also be connected to the pressure mechanisms thatcontrol the pressure applied by carrier head 140, to carrier headrotation motor 154 to control the carrier head rotation rate, to theplaten rotation motor 121 to control the platen rotation rate, or toslurry distribution system 130 to control the slurry compositionsupplied to the polishing pad. In addition, the computer 190 can beprogrammed to divide the measurements from the eddy current monitoringsystem 160 from each sweep beneath the substrate into a plurality ofsampling zones, to calculate the radial position of each sampling zone,and to sort the amplitude measurements into radial ranges, as discussedin U.S. Pat. No. 6,399,501. After sorting the measurements into radialranges, information on the film thickness can be fed in real-time into aclosed-loop controller to periodically or continuously modify thepolishing pressure profile applied by a carrier head in order to provideimproved polishing uniformity.

FIG. 3 illustrates an example of a substrate 10 that includes a stack302 of adjacent conductive layers on a wafer 313. A layer structure 311that can include other dielectric, conductive and/or semiconductivelayers can be positioned between the wafer 313 and the stack 302 ofconductive layers. Although illustrated as layers of uniform thicknessdeposited on a flat surface, the layers could be deposited on apatterned surface, and the layers themselves could be patterned.

The stack 302 can include at least two layers—an outer conductive layerand an adjacent underlying conductive layer. However, the stack couldinclude three, four or more layers. For example in FIG. 3, the outermostconductive layer 305 is on top of an underlying conductive layer 307,and the underlying conductive layer 307 is on top of another underlyingconductive layer 309. Adjacent layers will have different compositionsand different conductivities. For example, the outer layer can becopper, and the underlying layer can be a tantalum liner layer. Asanother example, the outer layer can be ruthenium, and the underlyinglayer can be a titanium nitride liner layer. As another example, theouter layer can be tungsten, and the underlying layer can be titaniumnitride liner layer. Adjacent layers can have different initialpre-polishing thicknesses. For example, the outermost layer 305 can bethinner, thicker or equally thick comparing to its underlying layer 307.For another more specific example, the outer layer 309 can be 200-1000 Åthick, whereas the underlying layer 311 can be 10-100 Å thick.

The last conductive layer to be polished in the stack, e.g., the bottomunderlying layer 309, can serve as a reference layer for monitoring in apolishing process, as discussed further below.

FIGS. 4A, 4B and 4C illustrate a process of in-situ real time profilecontrol for use in polishing a stack of adjacent conductive layers on asubstrate. Referring to FIG. 4A, the process can commence, e.g., at timeT₀, with polishing of the outer conductive layer.

The polishing process is monitored by the eddy current monitoring system160, and as noted above, the signals from the eddy current monitoringsystem 160 can be sorted into multiple zones based on the position ofthe measurement. The multiple zones can include a target zone and acontrol zone. The target zone can also be referred to as a first regionand the control zone can also be referred as a second region. For thein-situ real time profile controlling method, there may be more than onecontrol zone, for example, two to ten control zones. Each zone cancorrespond to one of the pressurizable chambers in the carrier head thatcontrols the down force on a portion of the substrate. Thus, the totalnumber of zone can be equal to the number of controllable chambers inthe carrier head, e.g., two to eleven.

For each zone of the multiple zones, the sequence of signal values forthat zone are converted into a sequence of characterizing values for thestack of adjacent conducting layers. The characterizing values canrepresent a total thickness of the stack of adjacent conducting layers,but without compensation for the fact that the different conductivelayers have different conductivities. In particular, the characterizingvalues can be calculated from the signal values using a staticcorrelation curve as shown in FIG. 2. The static correlation curve isbased on the conductivity of a reference layer. The reference layer canbe chosen as the last layer in the stack to be polished, e.g., thebottom layer in the stack of adjacent conductive layers on thesubstrate. Alternatively, the reference layer can be chosen as theanother layer in the stack to be polished, e.g., the first layer, thesecond layer, etc. However, as discussed below the correlation curve forthe same resistivity is used in calculation of the thickness of thestack of adjacent conductive layers on the substrate, regardless of thelayer currently being polished.

An in-situ real time profile control algorithm can use the sequence ofthickness measurements for each zone to adjust one or more polishingparameters, e.g., the pressure in one or more chambers in the carrierhead, to provide improved polishing uniformity. For example, the in-situreal time profile control algorithm can be configured to cause eachcontrol zone be closer to the desired thickness at the endpoint time(than without such adjustments).

In particular, for each zone, a function is fit to the sequence ofcharacterizing values for that zone. The example here shows first-orderlinear functions, e.g., lines 410 and 412, that are fit to each sequenceof characterizing values during polishing of the outer conductive layer.However, the fitting function doesn't have to be linear, for example, itcan be a higher-order polynomial or exponential function.

As shown in FIG. 4A, along the time axis (horizontal axis), three timepoints has been marked, they are the start time T₀, a pressure changetime T_(P1), and the first endpoint time T_(E1). The start time T₀ mayor may not be the exact starting time of a polishing process for theouter conductive layer on the stack of adjacent layers on a substrate.It can be a time when data starts to be collected from the in-situmonitoring system, or can represent the first data used for fitting ofthe function. The pressure change time T_(P1) represents a time at whichthe controller adjusts pressure when polishing the outer conductivelayer to attempt to improve polishing uniformity. Although FIG. 4Aillustrates a single pressure change time T_(P1), there can be aplurality of pressure adjustments. For example, pressure adjustments canoccur at a fixed frequency, e.g., every 2-30 platen rotations or every3-30 seconds.

The slope of the fitting functions, e.g., the lines 410 and 412, providecurrent polishing rates for the respective zones at the pressure changetime T_(P1). In this context, the “polishing rate” should be understoodas indicating the rate of change of the characterizing value, e.g., itcan, but need not, be expressed as a rate of change of the thicknessmeasurement. The current polishing rate k₁ for the target zone whenpolishing the outer conductive layer, can be used to project the targetzone thickness along the time axis, as shown by a first dashed line 416.The first endpoint time T_(E1) is calculated as the time that theprojected thickness of the target region reaches the targetcharacterizing value, e.g., target thickness, H_(TARGET). Similarly, thecurrent polishing rate k₃ for the control zone can be used to project acontrol zone thickness along the time axis, as shown by a second dashedline 418.

The controller 190 in FIG. 1A can determine the projected thicknessH_(CONTROL) of the control zone at the first endpoint time T_(E1) basedon the third polishing rate k₃. Assuming that the projected thickness ofthe control zone does not equal the target thickness H_(TARGET) at thefirst endpoint time T_(E1), the controller can calculate a first desiredpolishing rate k_(D1), e.g., shown by the slope of the dashed line 414,to bring the target zone and control zone to the target thickness atsubstantially the same time. The measured thickness of the control zoneH_(MEASURE) at the pressure change time T_(P1), as shown in the FIG. 3A,can be used to calculate of the desired polishing rate k_(D1), forexample according to the following equation:

$k_{D\; 1} = \frac{H_{MEASURE} - H_{TARGET}}{T_{P\; 1} - T_{E\; 1}}$

The controller can adjust a polishing parameter, e.g., the pressure inthe chambers associated with the control zone, to change the currentthird polishing rate k₃ to the first desired polishing rate k_(D1).Adjustments of chamber pressure in each chamber 146 a, 146 b and 146 cto bring the polishing rate of the control zone to the desired polishingrate can be calculated using a Preston matrix associated with eachconductive layer.

FIG. 4B illustrates an example of detecting exposure of the underlyingconductive layer during polishing. The detection of transition time ismarked in the time axis as T_(D1). The transition point in the plot ofthickness of the first region is marked by 420 at the time T_(D1). Inbrief, because of the difference in conductivity between the outer layerand the underlying layer, even if the physical polishing rate remainsconstant there will be a change in the apparent polishing rate. Inparticular, the controller can be configured to detect a change in theapparent polishing rate.

In some implementations, as shown by the flow diagram 500 in FIG. 5A,detection of exposure of and commencement of polishing of an underlyingconductive layer can be performed by obtaining a running time window ofa sequence of characterizing values at the target region through thein-situ monitoring system 160 (step 502), calculating a first trialpolishing rate k_(t1) based on a first portion of the sequence ofcharacterizing values within the time window (step 504), calculating asecond trial polishing rate k_(t2) based on the rest of the sequence ofcharactering values within the time window (step 506), and determining aslope change by comparing the second trial polishing rate k_(t2) withthe first trial polishing rate k_(t1) (step 508). The method shown inthe flow diagram 500 may be implemented for a plurality of times beforethe end of polishing.

Some other implementations, as shown by a flow diagram 510 in the FIG.5B, include a window logic to detect a deflection point of the polishingrate of the target zone. In this technique, a window logic is used toanalyze a sequence of characterizing values within the time window. Theanalysis can be performed by obtaining a sequence of characterizing datain a running time window (step 512), assigning an exit mode for thewindow logic based on information of the outer layer and the underlyinglayer (step 514), determining a slope change in a window logic with theexit mode (step 516), and determining the exposure of the underlyingconductive layer (or a polishing transition)(step 518). The informationof the outer layer and the underlying layer can be the relation betweena nominal rate of the outer layer r₁ and a nominal rate of theunderlying layer r₂. The exit mode of a window logic can be chosen byconsidering the relation of r₁ and r₂ in a manner as, for example, ifr₂>r₁, there will be a polishing slope increase at a polishingtransition, thus a bottom exit mode can be chosen in the window logic,for another example, if r₂≤r₁, there will be a polishing slope decreaseat a polishing transition, thus a right exit mode can be chosen in thewindow logic. The nominal rate r₁ and r₂ can be calculated with in-situmonitoring system under open-loop polishing each layer on a substrateindependently.

The nominal rate of r₁ and r₂ and the target polishing rate k₁ and k₂ ofeach layer are related through a preset down force F_(D) on the carrierhead 140, as below,

${r_{1} = \frac{k_{1}}{F_{D}}},{{{and}\mspace{14mu} r_{2}} = {\frac{k_{2}}{F_{D}}.}}$

The window logic may have predetermined parameters such as a time windowand a height window. Adjustments of the predetermined parameters of thewindow logic may be needed to reflect a deflection point in a polishingprofile of a region. The number of windows in the window logic is atleast one or above.

Either transition detection method can work for a stack of adjacentconducting layers on a substrate where the physical polishing rate of anouter layer and an underlying layer turn to be equal, so long as thereis a change in the apparent polishing rate due to the compositions ofthe outer layer and the underlying layer being different.

Upon the detection of a transition in the polishing profile of a targetzone, in some implementations, a second control algorithm can commencecontrol of the polishing process. In some implementations, the secondcontrol algorithm performs the same set of operations as the firstcontrol algorithm, but uses different values for the control variables.Examples of control variables having values that can be changed betweenthe first and second control algorithm include the nominal polishingrate of the layer being polished, the Preston matrix used to convertpolishing control parameters into polishing rates (and vice versa), thewindow logic for the current polishing step, e.g., the exit mode for thewindow logic, the pressure change frequency, the pressure update limits,the noise matrices, the rate estimate algorithms, the zone offsets, andthe gain.

For example, the Preston matrix can change from a first decoupleddiagonal matrix having a first set of values of along the diagonal to asecond decoupled diagonal matrix having a different second set of valuesalong the diagonal. This would be appropriate if the underlying layerdoes not have the same response to pressure as the outer layer. Forexample, if the underlying layer is a harder substance, it may be lessresponsive to changes in pressure, so larger values may be needed in thePreston matrix to account for the pressure change needed to provide adesired pressure. As another example, the Preston matrix can change froma decoupled diagonal matrix to an off-diagonal coupled matrix.

As another example, the adjustment frequency for chamber pressure canincrease or decrease. This would be appropriate if the underlying layerwill take more or less time to polishing than the outer layer. Forexample, if the underlying layer is expected to take less time topolishing than the outer layer, the adjustment frequency of chamberpressure can be increased to ensure that there are a sufficient numberof pressure modifications for the control zone to reliably match thetarget zone thickness.

As another example, the exit mode for a window logic can change from aright side exit mode to a bottom exit mode, or vice versa. For example,if the apparent polishing rate upon exposure of the additionalunderlying layer will be higher, then the exit mode of the window logiccan be changed to a bottom exit mode. If the apparent polishing rateupon exposure of the additional underlying layer will be lower, then theexit mode of the window logic can be changed to a right side exit mode.

The pressure update limits refer to the maximum amount that the pressurein the carrier head is permitted to change at each pressure change.Having the controller limit the pressure changes can preventover-correction and artificially induced oscillations in the polishingrates. Changing the pressure update limit would be appropriate if theunderlying layer will take more or less time to polishing than the outerlayer. For example, if the underlying layer is expected to take lesstime to polishing than the outer layer, the pressure update limit can beincreased to ensure that, given the lower number of modifications thatwill occur, the pressure will change sufficiently for the control zoneto reliably match the target zone thickness.

The rate estimate algorithm refers to the specific function that is usedto calculate the polishing rate. In particular, in calculating thepolishing rate, the controller typically uses a running window, e.g., acertain number of the most recent characterizing values. Changing thesize of the running window, e.g., the number of the most recentcharacterizing values used to calculate the polishing rate, would beappropriate if the signal generated during polishing the underlyinglayer has more or less noise than the signal generated during polishingof the outer layer. For example, if signal generated during polishing ofthe underlying layer is noisier, the size of the running window can beincreased to provide a longer averaging time.

In some implementations, the second control algorithm starts to collectcharacterizing values, calculate a polishing rate of a target zone, andproject an endpoint time as early as when the polishing process startsin the outer conductive layer, e.g., at T₀. However, the second controlalgorithm works independently of the first control algorithm, and thecontroller does not actually pass control of the current polishingprocess from the first control algorithm to the second control algorithmuntil a polishing transition (e.g., at T_(D1)) is detected plus asleeping time T_(s). The sleeping time T_(s) can be a fraction of asecond or more. The method of running two control algorithmssimultaneously and independently can ensure a seamless controltransition upon a polishing transition being detected, as a controlalgorithm needs some time to calculate the target polishing rate for thecurrent layer under polishing.

FIG. 4C illustrates the multiple pressure control method when polishingon the underlying conductive layer on a substrate. Similarly, theexample here shows first-order linear functions, e.g. lines 422 and 424,that are fit to each sequence of characterizing values during polishingof the underlying conductive layer. The fitting functions doesn't haveto be linear, as illustrated earlier in FIG. 4A.

As shown in FIG. 4C, another pressure change time T_(P2) and the secondendpoint time T_(E2) are marked along the time axis. The pressure changetime T_(P2) represents a time at which the controller adjusts pressurewhen polishing the underlying conductive layer to attempt to improvepolishing uniformity. Although FIG. 4C illustrates a single pressurechange time T_(P2), there can be a plurality of pressure adjustments.For example, pressure adjustments can occur at a fixed frequency, e.g.,every 2-30 platen rotations or every 3-30 seconds.

The slope of the fitting functions, e.g., lines 422 and 424, providecurrent polishing rates for respective zones at the pressure change timeT_(P2). The current polishing rate k₂ for the target zone when polishingthe underlying layer, can be used to project the target zone thicknessalong the time axis, as shown by a first dashed line 428. The secondendpoint time T_(E2) is calculated as the time that the projectedthickness of the target region reaches the target characterizing value,e.g., target thickness H_(TARGET). Similarly, the current polishing ratek₄ for the control zone, can be used to project a control zone thicknessalong the time axis, as shown by a second dashed line 430.

Similarly as described in the FIG. 4A, the controller 190 in FIG. 1A candetermine the projected polishing thickness H_(CONTROL) of the controlzone at the second endpoint time T_(E2) based on the fourth polishingrate k₄. Assuming that the control zone does not equal the targetthickness H_(TARGET) at the second endpoint time T_(E2), the controllercan calculate a second desired polishing rate k_(D2), e.g., shown by theslope of the black dashed line 426, to bring the target zone and controlzone to the target thickness at substantially the same time. Themeasured thickness of the control zone H_(MEASURE) at the pressurechange time T_(P2), as shown in the FIG. 3C, can be used to calculate ofthe desired polishing rate k_(D2) through the following equation:

$k_{D\; 2} = {\frac{H_{MEASURE} - H_{TARGET}}{T_{P\; 2} - T_{E\; 2}}.}$

The controller can adjust a polishing parameter, e.g., the pressure inthe chambers associated with the control zone, to change the currentfourth polishing rate k₄ to the second desired polishing rate k_(D2).Adjustments of chamber pressure in each chamber 146 a, 146 b and 146 cto bring the polishing rate of the control zone to the desired polishingrate can be calculated using a Preston matrix associated with eachconductive layer.

The controller 190 and its functional operations, such as the in-situreal time profile control algorithm, described in this specification canbe implemented in digital electronic circuitry, in tangibly-embodiedcomputer software or firmware, in computer hardware, including thestructures disclosed in this specification and their structuralequivalents, or in combinations of one or more of them. Embodiments ofthe subject matter described in this specification can be implemented asone or more computer programs, i.e., one or more modules of computerprogram instructions encoded on a tangible non transitory storage mediumfor execution by, or to control the operation of, data processingapparatus. Alternatively or in addition, the program instructions can beencoded on an artificially generated propagated signal, e.g., acomputer-generated electrical, optical, or electromagnetic signal, thatis generated to encode information for transmission to suitable receiverapparatus for execution by a data processing apparatus. The computerstorage medium can be a computer-readable storage device, acomputer-readable storage substrate, a random or serial access memorydevice, or a combination of one or more of them.

The term “data processing apparatus” refers to data processing hardwareand encompasses all kinds of apparatus, devices, and machines forprocessing data, including by way of example a programmable digitalprocessor, a digital computer, or multiple digital processors orcomputers. The apparatus can also be or further include special purposelogic circuitry, e.g., an FPGA (field programmable gate array) or anASIC (application specific integrated circuit). The apparatus canoptionally include, in addition to hardware, code that creates anexecution environment for computer programs, e.g., code that constitutesprocessor firmware, a protocol stack, a database management system, anoperating system, or a combination of one or more of them.

A computer program, which may also be referred to or described as aprogram, software, a software application, a module, a software module,a script, or code, can be written in any form of programming language,including compiled or interpreted languages, or declarative orprocedural languages, and it can be deployed in any form, including as astand alone program or as a module, component, subroutine, or other unitsuitable for use in a computing environment. A computer program may, butneed not, correspond to a file in a file system. A program can be storedin a portion of a file that holds other programs or data, e.g., one ormore scripts stored in a markup language document, in a single filededicated to the program in question, or in multiple coordinated files,e.g., files that store one or more modules, sub programs, or portions ofcode. A computer program can be deployed to be executed on one computeror on multiple computers that are located at one site or distributedacross multiple sites and interconnected by a data communicationnetwork.

The processes and logic flows described in this specification can beperformed by one or more programmable computers executing one or morecomputer programs to perform functions by operating on input data andgenerating output. The processes and logic flows can also be performedby, and apparatus can also be implemented as, special purpose logiccircuitry, e.g., an FPGA (field programmable gate array) or an ASIC(application specific integrated circuit). For a system of one or morecomputers to be “configured to” perform particular operations or actionsmeans that the system has installed on it software, firmware, hardware,or a combination of them that in operation cause the system to performthe operations or actions. For one or more computer programs to beconfigured to perform particular operations or actions means that theone or more programs include instructions that, when executed by dataprocessing apparatus, cause the apparatus to perform the operations oractions.

Computers suitable for the execution of a computer program include, byway of example, can be based on general or special purposemicroprocessors or both, or any other kind of central processing unit.Generally, a central processing unit will receive instructions and datafrom a read only memory or a random access memory or both. The essentialelements of a computer are a central processing unit for performing orexecuting instructions and one or more memory devices for storinginstructions and data. Generally, a computer will also include, or beoperatively coupled to receive data from or transfer data to, or both,one or more mass storage devices for storing data, e.g., magnetic,magneto optical disks, or optical disks. However, a computer need nothave such devices. Moreover, a computer can be embedded in anotherdevice, e.g., a mobile telephone, a personal digital assistant (PDA), amobile audio or video player, a game console, a Global PositioningSystem (GPS) receiver, or a portable storage device, e.g., a universalserial bus (USB) flash drive, to name just a few.

Computer readable media suitable for storing computer programinstructions and data include all forms of non volatile memory, mediaand memory devices, including by way of example semiconductor memorydevices, e.g., EPROM, EEPROM, and flash memory devices; magnetic disks,e.g., internal hard disks or removable disks; magneto optical disks; andCD ROM and DVD-ROM disks. The processor and the memory can besupplemented by, or incorporated in, special purpose logic circuitry.

Control of the various systems and processes described in thisspecification, or portions of them, can be implemented in a computerprogram product that includes instructions that are stored on one ormore non-transitory computer-readable storage media, and that areexecutable on one or more processing devices. The systems described inthis specification, or portions of them, can be implemented as anapparatus, method, or electronic system that may include one or moreprocessing devices and memory to store executable instructions toperform the operations described in this specification.

When a component, e.g., a controller, circuit, etc., is described orclaimed as “configured to” perform a task, this indicates that thecomponent includes structure (e.g., processor and non-transitorycomputer readable media having instructions encoded therein, ASICdevices, circuitry hardware, etc.) that performs that task duringoperation, not merely that the component could be modified, e.g.,programmed, to perform the task. The component can be said to beconfigured to perform the task even when the specified component is notcurrently operational (e.g., is not on).

A number of embodiments of the invention have been described.Nevertheless, it will be understood that various modifications may bemade without departing from the spirit and scope of the invention.

For example, the monitoring system can be used in a variety of polishingsystems. Either the polishing pad, or the carrier head, or both can moveto provide relative motion between the polishing surface and thesubstrate. The polishing pad can be a circular (or some other shape) padsecured to the platen, a tape extending between supply and take-uprollers, or a continuous belt. The polishing pad can be affixed on aplaten, incrementally advanced over a platen between polishingoperations, or driven continuously over the platen during polishing. Thepad can be secured to the platen during polishing, or there can be afluid bearing between the platen and polishing pad during polishing. Thepolishing pad can be a standard (e.g., polyurethane with or withoutfillers) rough pad, a soft pad, or a fixed-abrasive pad.

Although the discussion above focuses on a polishing system, thetechniques can be applied to other sorts of substrate processingsystems, e.g., deposition or etching systems, that include an eddycurrent in-situ monitoring system.

In addition, although the discussion above focuses on driving thecontrol zones to the same thickness, each zone can have a separatetarget thickness. In addition, rather than having a target zone and oneor more control zones, the system could be configured to have apredetermined desired endpoint time, and to drive each zone to itstarget thickness at the desired endpoint time (so each zone iseffectively a control zone).

Accordingly, other embodiments are within the scope of the followingclaims.

What is claimed is:
 1. A computer program product, comprising anon-transitory computer readable medium encoded with instructions tocause one or more processors to: method of controlling polishing,comprising: receive from an in-situ eddy current monitoring system asequence of characterizing values for a substrate being polished;repeatedly calculate a polishing rate from the sequence ofcharacterizing values during polishing of the substrate; for an initialtime period, calculate one or more first adjustments for one or morepolishing parameters based on a current polishing rate using a firstcontrol algorithm; detect a change in the polishing rate that meets atleast one first predetermined criterion that indicates exposure of anunderlying conductive layer in a stack of adjacent conductive layers onthe substrate that is being polished; and for a subsequent time periodafter detection of the change in the polishing rate, calculate one ormore second adjustments for one or more polishing parameters based onthe polishing rate using a different second control algorithm.
 2. Thecomputer program product, of claim 1, wherein the instructions torepeatedly calculate the polishing rate use a correlation curve based ona resistivity of a same layer from the stack of adjacent conductivelayers during the initial time period and the subsequent time period. 3.The computer program product of claim 1, wherein the instructions tocalculate the one or more first adjustments include instructions tocalculate a first projected time at which the substrate reaches a firsttarget value, and the instructions to calculate the one or more secondadjustments include instructions to calculate a second projected time atwhich the substrate reaches a second target value.
 4. The computerprogram product of claim 1, comprising instructions to iteratecalculation of a first polishing rate and calculation of the one or morefirst adjustments prior to detecting the change in the polishing rate ata first frequency, and instructions to iterate calculation of a secondpolishing rate and calculation of the one or more second adjustmentsafter to detecting the change in the polishing rate at a secondfrequency.
 5. The computer program product of claim 4, wherein the firstfrequency is different from the second frequency.
 6. The computerprogram product of claim 4, wherein the instructions to calculate thefirst polishing rate use a first function and the instructions tocalculate the second polishing rate use a different second function. 7.The computer program product of claim 6, wherein the first function andthe second function use running windows of different duration.
 8. Thecomputer program product of claim 1, wherein the instructions tocalculate the one or more first adjustments for the first polishingparameter use a first Preston matrix, and wherein the instructions tocalculate the one or more second adjustments for the second polishingparameter use a second Preston matrix.
 9. The computer program productof claim 8, wherein the first Preston matrix and second Preston matrixinclude different values for at least one element at the same row andcolumn in the first Preston matrix and second Preston matrix.
 10. Thecomputer program product of claim 1, comprising instructions to, afterdetection of the change in the polishing rate, detect a second change inthe polishing rate that meets at least one second predeterminedcriterion that indicates exposure of a further layer below theunderlying conductive layer.
 11. The computer program product of claim10, wherein the at least one first predetermined criterion and the atleast one second predetermined criterion are different criteria.
 12. Thecomputer program product of claim 11, wherein one of the first andsecond predetermined criteria is a window logic having a bottom exitmode, and another of the first and second predetermined criteria is awindow logic having a right side exit mode.
 13. The computer programproduct of claim 1, wherein the instructions to calculate the one ormore first adjustments comprise instructions to limit the firstadjustments to a first maximum change, and wherein the instructions tocalculate the one or more second adjustments comprise instructions tolimit the second adjustments to a different second maximum change. 14.The computer program product of claim 1, wherein the instructions tocalculate the polishing rate from the sequence of characterizing valuesuse a correlation curve based on a resistivity of a layer from the stackof adjacent conductive layers.
 15. The computer program product of claim15, wherein the layer is the last conductive layer from the stack ofadjacent conductive layers.
 16. A method of controlling polishing,comprising: polishing a stack of adjacent conductive layers on asubstrate, the stack of adjacent conductive layers including an outerconductive layer and an underlying conductive layer; measuring with anin-situ eddy current monitoring system a sequence of characterizingvalues for the substrate during polishing; during polishing, repeatedlycalculating a polishing rate from the sequence of characterizing values;for an initial time period, calculating one or more adjustments for oneor more polishing parameters based on a current polishing rate using afirst control algorithm; detecting a change in the polishing rate thatmeets at least one first predetermined criterion that indicates exposureof the underlying conductive layer; and for a subsequent time periodafter detecting the change in the polishing rate, calculating one ormore adjustments for one or more polishing parameters based on thepolishing rate using a different second control algorithm.
 17. Themethod of claim 16, wherein repeatedly calculating the polishing rateincludes using a correlation curve based on a resistivity of a samelayer from the stack of adjacent conductive layers during the initialtime period and the subsequent time period.
 18. A polishing system,comprising: a platen to support a polishing pad; a carrier head to holda substrate; a motor to generate relative motion between the platen andthe carrier head; an in-situ eddy current monitoring system to monitorthe substrate during polishing of the substrate; and a controllerconfigured to receive from the in-situ eddy current monitoring system asequence of characterizing values for a substrate being polished;repeatedly calculate a polishing rate from the sequence ofcharacterizing values during polishing of the substrate; for an initialtime period, calculate one or more adjustments for one or more polishingparameters based on a current polishing rate using a first controlalgorithm; detect a change in the polishing rate that meets at least onefirst predetermined criterion that indicates exposure of an underlyingconductive layer in a stack of adjacent conductive layers on thesubstrate that is being polished; and for a subsequent time period afterdetection of the change in the polishing rate, calculate one or moreadjustments for one or more polishing parameters based on the polishingrate using a different second control algorithm.
 19. The system of claim18, wherein the carrier head comprises a plurality of independentlycontrollable chambers, and the one or more polishing parameters includeone or more pressures for one or more of the chambers.